Method and apparatus for transmitting reference signal for channel change measurement in wireless communication system

ABSTRACT

Disclosed herein is a method for transmitting a reference signal from a base station to a user equipment (UE) in a wireless communication system. The method includes mapping the reference signal defined by a plurality of antenna ports to resource elements, applying precoding to the reference signal using different precoders according to the plurality of antenna ports, and transmitting the precoded reference signal to the UE.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2015/001425, filed on Feb. 12, 2015,which claims the benefit of U.S. Provisional Application No. 62/027,230,filed on Jul. 21, 2014, the contents of which are all herebyincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for transmitting areference signal for channel change measurement in a wirelesscommunication system.

BACKGROUND ART

As an example of a wireless communication system to which the presentinvention is applicable, a 3rd Generation Partnership Project (3GPP)Long Term Evolution (LTE) communication system will be schematicallydescribed.

FIG. 1 is a diagram showing a network structure of an Evolved UniversalMobile Telecommunications System (E-UMTS) as a wireless communicationsystem. The E-UMTS is an evolved form of the UMTS and has beenstandardized in the 3GPP. Generally, the E-UMTS may be called a LongTerm Evolution (LTE) system. For details of the technical specificationsof the UMTS and E-UMTS, refer to Release 7 and Release 8 of “3rdGeneration Partnership Project; Technical Specification Group RadioAccess Network”.

Referring to FIG. 1, the E-UMTS mainly includes a User Equipment (UE),base stations (or eNBs or eNode Bs), and an Access Gateway (AG) which islocated at an end of a network (E-UTRAN) and which is connected to anexternal network. Generally, an eNB can simultaneously transmit multipledata streams for a broadcast service, a multicast service and/or aunicast service.

One or more cells may exist per eNB. The cell is set to use a bandwidthsuch as 1.25, 2.5, 5, 10, 15 or 20 MHz to provide a downlink or uplinktransmission service to several UEs. Different cells may be set toprovide different bandwidths. The eNB controls data transmission orreception of a plurality of UEs. The eNB transmits downlink (DL)scheduling information of DL data so as to inform a corresponding UE oftime/frequency domain in which data is transmitted, coding, data size,and Hybrid Automatic Repeat and reQest (HARQ)-related information. Inaddition, the eNB transmits uplink (UL) scheduling information of ULdata to a corresponding UE so as to inform the UE of a time/frequencydomain which may be used by the UE, coding, data size and HARQ-relatedinformation. An interface for transmitting user traffic or controltraffic can be used between eNBs. A Core Network (CN) may include an AG,a network node for user registration of the UE, etc. The AG managesmobility of a UE on a Tracking Area (TA) basis. One TA includes aplurality of cells.

Although wireless communication technology has been developed up to LongTerm Evolution (LTE) based on Wideband Code Division Multiple Access(WCDMA), the demands and the expectations of users and providerscontinue to increase. In addition, since other radio access technologieshave been continuously developed, new technology evolution is requiredto secure high competitiveness in the future. Decrease in cost per bit,increase in service availability, flexible use of a frequency band,simple structure, open interface, suitable User Equipment (UE) powerconsumption and the like are required.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ina method and apparatus for transmitting a reference signal for channelchange measurement in a wireless communication system.

Technical Solution

The object of the present invention can be achieved by providing amethod for transmitting a reference signal from a base station to a userequipment (UE) in a wireless communication system including mapping thereference signal defined by a plurality of antenna ports to resourceelements, applying precoding to the reference signal using differentprecoders according to the plurality of antenna ports, and transmittingthe precoded reference signal to the UE.

The mapping the reference signal to the resource elements may includemapping the reference signal defined by the plurality of antenna portsto resource elements included in a first slot of one subframe, andrepeatedly mapping the reference signal defined by the plurality ofantenna ports to resource elements included in a second slot of thesubframe. In this case, for each of the plurality of antenna ports,subcarrier indices of the resource elements of the first slot, to whichthe reference signal is mapped, may be equal to subcarrier indices ofthe resource elements of the second slot, to which the reference signalis mapped.

In another aspect of the present invention, provided herein is a basestation, that is, a transmitter in a wireless communication systemincluding a processor configured to map a reference signal defined by aplurality of antenna ports to resource elements and apply precoding tothe reference signal using different precoders according to theplurality of antenna ports, and a wireless communication moduleconfigured to transmit the precoded reference signal to a receiver.

The processor may map the reference signal defined by the plurality ofantenna ports to resource elements included in a first slot of onesubframe, and repeatedly map the reference signal defined by theplurality of antenna ports to resource elements included in a secondslot of the subframe. For each of the plurality of antenna ports,subcarrier indices of the resource elements of the first slot, to whichthe reference signal is mapped, may be equal to subcarrier indices ofthe resource elements of the second slot, to which the reference signalis mapped.

In the above-described embodiments, the precoders may correspond torank 1. The resource elements may be included in only one of allresource blocks allocated to the UE or the receiver. In particular,information about the resource block is transmitted to the UE or thereceiver via a higher layer signal.

Advantageous Effects

According to embodiments of the present invention, in a wirelesscommunication system, a UE can efficiently measure and report channelchange information and a base station may more efficiently performbeamforming based on the channel change information.

It will be appreciated by persons skilled in the art that that theeffects that can be achieved through the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a network structure of an Evolved UniversalMobile Telecommunications System (E-UMTS) as an example of a wirelesscommunication system.

FIG. 2 is a diagram showing a control plane and a user plane of a radiointerface protocol architecture between a User Equipment (UE) and anEvolved Universal Terrestrial Radio Access Network (E-UTRAN) based on a3rd Generation Partnership Project (3GPP) radio access network standard.

FIG. 3 is a diagram showing physical channels used in a 3GPP system anda general signal transmission method using the same.

FIG. 4 is a diagram showing the structure of a radio frame used in aLong Term Evolution (LTE) system.

FIG. 5 is a diagram showing the structure of a downlink radio frame usedin an LTE system.

FIG. 6 is a diagram showing the structure of an uplink subframe used inan LTE system.

FIG. 7 is a diagram showing the configuration of a general multipleinput multiple output (MIMO) system.

FIG. 8 is a diagram illustrating an antenna tilting scheme.

FIG. 9 is a diagram comparing comparison between an existing antennasystem and an active antenna system.

FIG. 10 is a diagram showing an example of forming a UE-specific beambased on an active antenna system.

FIG. 11 is a diagram showing a 3-dimensional (3D) beam transmissionscenario based on an active antenna system.

FIG. 12 is a diagram showing comparison in beam coverage between anexisting MIMO transmission scheme and a BA beamforming scheme.

FIG. 13 is a diagram showing the concept of a DA beamforming scheme.

FIG. 14 is a diagram showing the features of a DA beamforming scheme.

FIG. 15 is a diagram showing the concept of a DBA beamforming scheme.

FIGS. 16 and 17 are diagrams showing examples of a pilot for channelmeasurement in a vertical direction.

FIGS. 18 and 19 are diagrams showing examples of a pilot for channelmeasurement in a horizontal direction.

FIG. 20 is a diagram showing an example of calculating a channelvariation value during a specific duration.

FIG. 21 is a diagram showing an example of measuring and reporting apilot index in a vertical/horizontal direction.

FIG. 22 is a diagram showing an example of measuring and reportingchannel variation in a vertical direction and a horizontal directionbased on a non-precoded pilot.

FIG. 23 is a diagram showing another example of measuring and reportinga pilot index in a vertical/horizontal direction.

FIG. 24 is a diagram showing an example of transmitting a referencesignal for channel change measurement according to an embodiment of thepresent invention.

FIG. 25 is a diagram showing an example of repeatedly transmitting anantenna port of a reference signal for channel change measurement at thesame time interval according to an embodiment of the present invention.

FIG. 26 is a block diagram of a communication apparatus according to oneembodiment of the present invention.

BEST MODE

The configuration, operation and other features of the present inventionwill be understood by the embodiments of the present invention describedwith reference to the accompanying drawings. The following embodimentsare examples of applying the technical features of the present inventionto a 3rd Generation Partnership Project (3GPP) system.

Although, for convenience, the embodiments of the present invention aredescribed using the LTE system and the LTE-A system in the presentspecification, the embodiments of the present invention are applicableto any communication system corresponding to the above definition. Inaddition, although the embodiments of the present invention aredescribed based on a Frequency Division Duplex (FDD) scheme in thepresent specification, the embodiments of the present invention may beeasily modified and applied to a Half-Duplex FDD (H-FDD) scheme or aTime Division Duplex (TDD) scheme.

In addition, in the present specification, the term “base station” mayinclude a remote radio head (RRH), an eNB, a transmission point (TP), areception point (RP), a relay, etc.

FIG. 2 shows a control plane and a user plane of a radio interfaceprotocol between a UE and an Evolved Universal Terrestrial Radio AccessNetwork (E-UTRAN) based on a 3GPP radio access network standard. Thecontrol plane refers to a path used for transmitting control messagesused for managing a call between the UE and the network. The user planerefers to a path used for transmitting data generated in an applicationlayer, e.g., voice data or Internet packet data.

A physical (PHY) layer of a first layer provides an information transferservice to a higher layer using a physical channel. The PHY layer isconnected to a Medium Access Control (MAC) layer located on a higherlayer via a transport channel. Data is transported between the MAC layerand the PHY layer via the transport channel. Data is also transportedbetween a physical layer of a transmitting side and a physical layer ofa receiving side via a physical channel. The physical channel uses atime and a frequency as radio resources. More specifically, the physicalchannel is modulated using an Orthogonal Frequency Division MultipleAccess (OFDMA) scheme in downlink and is modulated using aSingle-Carrier Frequency Division Multiple Access (SC-FDMA) scheme inuplink.

A Medium Access Control (MAC) layer of a second layer provides a serviceto a Radio Link Control (RLC) layer of a higher layer via a logicalchannel. The RLC layer of the second layer supports reliable datatransmission. The function of the RLC layer may be implemented by afunctional block within the MAC. A Packet Data Convergence Protocol(PDCP) layer of the second layer performs a header compression functionto reduce unnecessary control information for efficient transmission ofan Internet Protocol (IP) packet such as an IPv4 packet or an IPv6packet in a radio interface having a relatively small bandwidth.

A Radio Resource Control (RRC) layer located at the bottom of a thirdlayer is defined only in the control plane and is responsible forcontrol of logical, transport, and physical channels in association withconfiguration, re-configuration, and release of Radio Bearers (RBs). TheRB is a service that the second layer provides for data communicationbetween the UE and the network. To accomplish this, the RRC layer of theUE and the RRC layer of the network exchange RRC messages. The UE is inan RRC connected mode if an RRC connection has been established betweenthe RRC layer of the radio network and the RRC layer of the UE.Otherwise, the UE is in an RRC idle mode. A Non-Access Stratum (NAS)layer located above the RRC layer performs functions such as sessionmanagement and mobility management.

Downlink transport channels for transmission of data from the network tothe UE include a Broadcast Channel (BCH) for transmission of systeminformation, a Paging Channel (PCH) for transmission of paging messages,and a downlink Shared Channel (SCH) for transmission of user traffic orcontrol messages. Traffic or control messages of a downlink multicast orbroadcast service may be transmitted through a downlink SCH and may alsobe transmitted through a downlink multicast channel (MCH). Uplinktransport channels for transmission of data from the UE to the networkinclude a Random Access Channel (RACH) for transmission of initialcontrol messages and an uplink SCH for transmission of user traffic orcontrol messages. Logical channels, which are located above thetransport channels and are mapped to the transport channels, include aBroadcast Control Channel (BCCH), a Paging Control Channel (PCCH), aCommon Control Channel (CCCH), a Multicast Control Channel (MCCH), and aMulticast Traffic Channel (MTCH).

FIG. 3 is a diagram showing physical channels used in a 3GPP system anda general signal transmission method using the same.

A UE performs an initial cell search operation such as synchronizationwith an eNB when power is turned on or the UE enters a new cell (S301).The UE may receive a Primary Synchronization Channel (P-SCH) and aSecondary Synchronization Channel (S-SCH) from the eNB, performsynchronization with the eNB, and acquire information such as a cell ID.Thereafter, the UE may receive a physical broadcast channel from the eNBso as to acquire broadcast information within the cell. Meanwhile, theUE may receive a Downlink Reference Signal (DL RS) so as to confirm adownlink channel state in the initial cell search step.

The UE, which has completed the initial cell search, may receive aPhysical Downlink Control Channel (PDCCH) and a Physical Downlink SharedChannel (PDSCH) according to information included in the PDCCH so as toacquire more detailed system information (S302).

Meanwhile, if the eNB is initially accessed or radio resources forsignal transmission are not present, the UE may perform a Random AccessProcedure (RACH) (step S303 to S306) with respect to the eNB. In thiscase, the UE may transmit a specific sequence through a Physical RandomAccess Channel (PRACH) as a preamble (S303 and S305), and receive aresponse message of the preamble through the PDCCH and the PDSCHcorresponding thereto (S304 and S306). In the case of contention-basedRACH, a contention resolution procedure may be further performed.

The UE, which has performed the above procedures, may performPDCCH/PDSCH reception (S307) and Physical Uplink Shared ChannelPUSCH)/Physical Uplink Control Channel (PUCCH) transmission (S308) as ageneral uplink/downlink signal transmission procedure. In particular,the UE receives downlink control information (DCI) through a PDCCH.Here, the DCI includes control information such as resource allocationinformation of the UE and the format thereof differs according to theuse purpose.

The control information transmitted from the UE to the eNB in uplink ortransmitted from the eNB to the UE in downlink includes adownlink/uplink ACK/NACK signal, a Channel Quality Indicator (CQI), aPrecoding Matrix Index (PMI), a Rank Indicator (RI), and the like. Inthe case of the 3GPP LTE system, the UE may transmit the controlinformation such as CQI/PMI/RI through the PUSCH and/or the PUCCH.

FIG. 4 is a diagram showing the structure of a radio frame used in aLong Term Evolution (LTE) system.

Referring to FIG. 4, the radio frame has a length of 10 ms(327200×T_(s)) and includes 10 subframes with the same size. Each of thesubframes has a length of 1 ms and includes two slots. Each of the slotshas a length of 0.5 ms (15360×T_(s)). T_(s) denotes a sampling time, andis represented by T_(s)=1/(15 kHz×2048)=3.2552×10⁻⁸ (about 33 ns). Eachslot includes a plurality of OFDM symbols in a time domain, and includesa plurality of resource blocks (RBs) in a frequency domain. In the LTEsystem, one RB includes 12 subcarriers×7(6) OFDM or SC-FDMA symbols. ATransmission Time Interval (TTI) which is a unit time for transmissionof data may be determined in units of one or more subframes. Thestructure of the radio frame is only exemplary and the number ofsubframes included in the radio frame, the number of slots included inthe subframe, or the number of OFDM symbols included in the slot may bevariously changed.

FIG. 5 is a diagram showing a control channel included in a controlregion of one subframe in a downlink radio frame.

Referring to FIG. 5, a subframe includes 14 OFDM symbols. The first tothird OFDM symbols are used as a control region and the remaining 13 to11 OFDM symbols are used as a data region, according to subframeconfiguration. In FIG. 5, R1 to R4 denote reference signals (RS) orpilot signals for antennas 0 to 3. The RS is fixed to a constant patternwithin a subframe regardless of the control region and the data region.A control channel is allocated to resources, to which the RS is notallocated, in the control region, and a traffic channel is alsoallocated to resources, to which the RS is not allocated, in the controlregion. Examples of the control channel allocated to the control regioninclude a Physical Control Format Indicator Channel (PCFICH), a PhysicalHybrid-ARQ Indicator Channel (PHICH), a Physical Downlink ControlChannel (PDCCH), etc.

The Physical Control Format Indicator Channel (PCFICH) informs the UE ofthe number of OFDM symbols used for the PDCCH per subframe. The PCFICHis located at a first OFDM symbol and is configured prior to the PHICHand the PDCCH. The PCFICH includes four Resource Element Groups (REGs)and the REGs are dispersed in the control region based on a cellidentity (ID). One REG includes four resource elements (REs). The PCFICHhas a value of 1 to 3 or 2 to 4 according to bandwidth and is modulatedusing a Quadrature Phase Shift Keying (QPSK) scheme.

The Physical Hybrid-ARQ Indicator Channel (PHICH) is used to carry HARQACK/NACK for uplink transmission. That is, the PHICH refers to a channelvia which DL ACK/NACK information for uplink HARQ is transmitted. ThePHICH includes one REG and is scrambled on a cell-specific basis.ACK/NACK is indicated by one bit and is modulated using a binary phaseshift keying (BPSK) scheme. The modulated ACK/NACK is repeatedly spreadwith a spreading factor (SF) of 2 or 4. A plurality of PHICHs mapped tothe same resources configures a PHICH group. The number of PHICHsmultiplexed in the PHICH group is determined according to the number ofspreading codes. The PHICH (group) is repeated three times in order toobtain diversity gain in a frequency region and/or time region.

The Physical Downlink Control Channel (PDCCH) is allocated to the firstn OFDM symbols of a subframe. Here, n is an integer of 1 or more and isindicated by a PCFICH. The PDCCH includes one or more Control ChannelElements (CCEs). The PDCCH informs each UE or a UE group of informationassociated with resource allocation of a Paging Channel (PCH) and aDownlink-Shared Channel (DL-SCH), both of which are transport channels,uplink scheduling grant, HARQ information, etc. The paging channel (PCH)and the downlink-shared channel (DL-SCH) are transmitted through aPDSCH. Accordingly, the eNB and the UE transmit and receive data throughthe PDSCH except for specific control information or specific servicedata.

Information indicating to which UE (one or a plurality of UEs) data ofthe PDSCH is transmitted and information indicating how the UEs receiveand decode the PDSCH data are transmitted in a state of being includedin the PDCCH. For example, it is assumed that a specific PDCCH isCRC-masked with a Radio Network Temporary Identity (RNTI) “A”, andinformation about data transmitted using radio resource (e.g., frequencylocation) “B” and transmission format information (e.g., transmissionblock size, modulation scheme, coding information, or the like) “C” istransmitted via a specific subframe. In this case, one or more UEslocated within a cell monitor a PDCCH using its own RNTI information,and if one or more UEs having “A” RNTI are present, the UEs receive thePDCCH and receive the PDSCH indicated by “B” and “C” through theinformation about the received PDCCH.

FIG. 6 is a diagram showing the structure of an uplink subframe used inan LTE system.

Referring to FIG. 6, an uplink subframe may be divided into a region towhich a Physical Uplink Control Channel (PUCCH) carrying uplink controlinformation is allocated and a region to which a Physical Uplink SharedChannel (PUSCH) carrying user data is allocated. A middle portion of thesubframe is allocated to the PUSCH and both sides of a data region in afrequency domain are allocated to the PUCCH. Uplink control informationtransmitted on the PUCCH includes an ACK/NACK signal used for HARQ, aChannel Quality Indicator (CQI) indicating a downlink channel status, arank indicator (RI) for MIMO, a scheduling request (SR) which is anuplink radio resource allocation request, etc. The PUCCH for one UE usesone resource block occupying different frequencies in slots within thesubframe. Two slots use different resource blocks (or subcarriers)within the subframe. That is, two resource blocks allocated to the PUCCHare frequency-hopped in a slot boundary. FIG. 6 shows the case in whicha PUCCH having m=0, a PUCCH having m=1, a PUCCH having m=2, and a PUCCHhaving m=3 are allocated to the subframe.

Hereinafter, a Multiple-Input Multiple-Output (MIMO) system will bedescribed. In the MIMO system, multiple transmission antennas andmultiple reception antennas are used. By this method, datatransmission/reception efficiency can be improved. That is, since aplurality of antennas is used in a transmitter or a receiver of awireless communication system, capacity can be increased and performancecan be improved. Hereinafter, MIMO may also be called “multi-antenna”.

In the multi-antenna technique, a single antenna path is not used forreceiving one message. Instead, in the multi-antenna technique, datafragments received via several antennas are collected and combined so asto complete data. If the multi-antenna technique is used, a datatransfer rate may be improved within a cell region having a specificsize or system coverage may be increased while ensuring a specific datatransfer rate. In addition, this technique may be widely used in amobile communication terminal, a repeater and the like. According to themulti-antenna technique, it is possible to overcome a limit intransmission amount of conventional mobile communication using a singleantenna.

The configuration of the general multi-antenna (MIMO) communicationsystem is shown in FIG. 7. N_(T) transmission antennas are provided in atransmitter and N_(R) reception antennas are provided in a receiver. Ifthe multiple antennas are used in both the transmitter and the receiver,theoretical channel transmission capacity is increased as compared withthe case where multiple antennas are used in only one of the transmitteror the receiver. The increase in the channel transmission capacity isproportional to the number of antennas. Accordingly, transfer rate isimproved and frequency efficiency is improved. If a maximum transferrate in the case where one antenna is used is R_(o), a transfer rate inthe case where multiple antennas are used can be theoretically increasedby a value obtained by multiplying R_(o) by a rate increase ratio R_(i)as shown in Equation 1 below. Here, R_(i) is the smaller of the twovalues N_(T) and N_(R).R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For example, in a MIMO system using four transmit antennas and fourreception antennas, it is possible to theoretically acquire a transferrate which is four times that of a single antenna system. After thetheoretical increase in the capacity of the MIMO system was proved inthe mid-1990s, various technologies of substantially improving a datatransmission rate have been actively developed up to now. In addition,several technologies are already applied to the various radiocommunication standards such as the third-generation mobilecommunication and the next-generation wireless local area network (LAN).

According to the researches into the MIMO antenna up to now, variousresearches such as researches into information theory related to thecomputation of the communication capacity of a MIMO antenna in variouschannel environments and multiple access environments, researches intothe model and the measurement of the radio channels of the MIMO system,and researches into space-time signal processing technologies ofimproving transmission reliability and transmission rate have beenactively conducted.

The communication method of the MIMO system will be described in moredetail using mathematical modeling. As shown in FIG. 7, it is assumedthat NT transmit antennas and NR reception antennas are present. Intransmitted signals, if the NT transmit antennas are present, the numberof pieces of maximally transmittable information is NT. The transmittedinformation may be expressed by a vector shown in Equation 2 below.s=└s ₁ ,s ₂ , . . . ,s _(N) _(T) ┘^(T)  [Equation 2]

The transmitted information s₁, s₂, . . . , s_(N) _(T) may havedifferent transmit powers. If the respective transmit powers are P₁, P₂,. . . , P_(N) _(T) , the transmitted information with adjusted powersmay be expressed by a vector shown in Equation 3 below.ŝ=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T) =[P ₁ s ₁ ,P ₂ s ₂ , . . . ,P_(N) _(T) s _(N) _(T) ]^(T)  [Equation 2]

In addition, ŝ may be expressed using a diagonal matrix P of thetransmit powers as shown in Equation 4 below.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Considers that the N_(T) actually transmitted signals x₁, x₂, . . . ,x_(N) _(T) are configured by applying a weight matrix W to theinformation vector ŝ with the adjusted transmit powers. The weightmatrix serves to appropriately distribute the transmitted information toeach antenna according to a transport channel state, etc. Suchtransmitted signals x₁, x₂, . . . , x_(N) _(T) may be expressed by usinga vector X as shown in Equation 5 below. Wij denotes a weight between ani-th transmit antenna and j-th information. W is also called a weightmatrix or a precoding matrix.

$\begin{matrix}{x = {\quad{\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{1} \\\vdots \\x_{N_{T}}\end{bmatrix} = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{1N_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {W\hat{s}{\quad{= {WPs}}}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

In general, the physical meaning of the rank of the channel matrix maybe a maximum number of elements capable of transmitting differentinformation via a given channel. Accordingly, since the rank of thechannel matrix is defined as the smaller of the number of independentrows or columns, the rank of the matrix is not greater than the numberof rows or columns. The rank rank(H) of the channel matrix H ismathematically expressed by Equation 6.rank(H)≤min(N _(T) ,N _(R))  [Equation 6]

In addition, different information transmitted using the MIMO technologyis defined as “transmitted stream” or “stream”. Such “stream” may bereferred to as “layer”. Then, the number of transmitted streams is notgreater than the rank which is a maximum number capable of transmittingdifferent information. Accordingly, the channel rank H is expressed byEquation 7 below.# of streams≤rank(H)≤min(N _(T) ,N _(R))  [Equation 7]

where, “# of streams” denotes the number of streams. It should be notedthat one stream may be transmitted via one or more antennas.

There are various methods for associating one or more streams withseveral antennas. These methods will be described according to the kindof the MIMO technology. A method of transmitting one stream via severalantennas is referred to as a spatial diversity method and a method oftransmitting several streams via several antennas is referred to as aspatial multiplexing method. In addition, a hybrid method which is acombination of the spatial diversity method and the spatial multiplexingmethod may be used.

Now, a description of a Channel State Information (CSI) report is given.In the current LTE standard, a MIMO transmission scheme is categorizedinto open-loop MIMO operated without CSI and closed-loop MIMO operatedbased on CSI. Especially, according to the closed-loop MIMO system, eachof the eNB and the UE may be able to perform beamforming based on CSI toobtain a multiplexing gain of MIMO antennas. To obtain CSI from the UE,the eNB allocates a PUCCH or a PUSCH to command the UE to feed back CSIfor a downlink signal.

CSI is divided into three types of information: a Rank Indicator (RI), aPrecoding Matrix Index (PMI), and a Channel Quality Indicator (CQI).First, RI is information on a channel rank as described above andindicates the number of streams that can be received via the sametime-frequency resource. Since RI is determined by long-term fading of achannel, it may be generally fed back at a period longer than that ofPMI or CQI.

Second, PMI is a value reflecting a spatial characteristic of a channeland indicates a precoding matrix index of the eNB preferred by the UEbased on a metric of Signal-to-Interference plus Noise Ratio (SINR).Lastly, CQI is information indicating the strength of a channel andindicates a reception SINR obtainable when the eNB uses PMI.

In an evolved communication system such as an LTE-A system, multi-userdiversity using Multi-User MIMO (MU-MIMO) is additionally obtained.Since interference between UEs multiplexed in an antenna domain existsin the MU-MIMO scheme, CSI accuracy may greatly affect not onlyinterference of a UE that has reported CSI but also interference ofother multiplexed UEs. Hence, in order to correctly perform MU-MIMOoperation, it is necessary to report CSI having accuracy higher thanthat of a Single User-MIMO (SU-MIMO) scheme.

Accordingly, LTE-A standard has determined that a final PMI should beseparately designed into W1, which a long-term and/or wideband PMI, andW2, which is a short-term and/or subband PMI.

An example of a hierarchical codebook transform scheme configuring onefinal PMI from among W1 and W2 may use a long-term covariance matrix ofa channel as indicated in Equation 8:W=norm(W1W2)  [Equation 8]

In Equation 8, W2 of a short-term PMI indicates a codeword of a codebookconfigured to reflect short-term channel information, W denotes acodeword of a final codebook, and norm(A) indicates a matrix in which anorm of each column of a matrix A is normalized to 1.

The detailed configurations of W1 and W2 are shown in Equation 9:

$\begin{matrix}{{{W\; 1(i)} = \begin{bmatrix}X_{i} & 0 \\0 & X_{i}\end{bmatrix}},{{where}\mspace{14mu} X_{i}\mspace{14mu}{is}\mspace{14mu}{{Nt}/2}\mspace{14mu}{by}\mspace{14mu} M\mspace{14mu}{{matrix}.}}} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack \\{{{W\; 2(j)} = {\overset{\overset{r\mspace{14mu}{columns}}{︷}}{\begin{bmatrix}e_{M}^{k} & e_{M}^{l} & \; & e_{M}^{m} \\\; & \; & \cdots & \; \\{\alpha_{j}e_{M}^{k}} & {\beta_{j}e_{M}^{l}} & \; & {\gamma_{j}e_{M}^{m}}\end{bmatrix}}\left( {{{if}\mspace{14mu}{rank}} = r} \right)}},{{{where}\mspace{14mu} I} \leq k},l,{m \leq {M\mspace{14mu}{and}\mspace{14mu} k}},l,{m\mspace{14mu}{are}\mspace{14mu}{{integer}.}}} & \;\end{matrix}$

In Equation 9, the codebook configurations are designed to reflectchannel correlation properties generated when cross polarized antennasare used and when a space between antennas is dense, for example, when adistance between adjacent antennas is less than a′half of signalwavelength. The cross polarized antennas may be categorized into ahorizontal antenna group and a vertical antenna group. Each antennagroup has the characteristic of a Uniform Linear Array (ULA) antenna andthe two groups are co-located.

Accordingly, a correlation between antennas of each group hascharacteristics of the same linear phase increment and a correlationbetween antenna groups has characteristics of phase rotation.Consequently, since a codebook is a value obtained by quantizing achannel, it is necessary to design a codebook such that characteristicsof a channel are reflected. For convenience of description, a rank-1codeword generated by the aforementioned configurations is shown asfollows:

$\begin{matrix}{{W\; 1(i)*W\; 2(j)} = \begin{bmatrix}{X_{i}(k)} \\{\alpha_{j}{X_{i}(k)}}\end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

In Equation 10, a codeword is expressed as a vector of N_(T)×1 (where NTis the number of Tx antennas) and is structured with an upper vectorX_(i)(k) and a lower vector α_(j)X_(i)(k) which show correlationcharacteristics of a horizontal antenna group and a vertical antennagroup, respectively. X_(i) ^((k)) is preferably expressed as a vectorhaving the characteristics of linear phase increment by reflecting thecharacteristics of a correlation between antennas of each antenna groupand may be a DFT matrix as a representative example.

Hereinafter, an active antenna system (AAS) and three-dimensional (3D)beamforming will be described.

In an existing cellular system, a base station has used a method forreducing inter-cell interference (ICI) using mechanical tilting orelectrical tilting and improving throughput, e.g., signal tointerference plus noise ratios (SINRs), of UEs of a cell, which will bedescribed in greater detail with reference to the drawings.

FIG. 8 is a diagram illustrating an antenna tilting method. Inparticular, FIG. 8(a) shows an antenna structure to which antennatilting is not applied, FIG. 8(b) shows an antenna structure to whichmechanical tilting is applied, and FIG. 8(c) shows an antenna structureto which both mechanical tilting and electrical tilting are applied.

In comparison of FIG. 8(a) with FIG. 8(b), when mechanical tilting isapplied, a beam direction is fixed upon initial installation as shown inFIG. 8(b). Further, when electrical tilting is applied, as shown in FIG.8(c), a tilting angle may be changed using an internal phase shiftmodule but only restrictive vertical beamforming is possible due tofixed tilting.

FIG. 9 is a diagram showing comparison between an existing antennasystem and an active antenna system. In particular, FIG. 9(a) shows anexisting antenna system and FIG. 9(b) shows an active antenna system.

Referring to FIG. 9, unlike the existing antenna system, the activeantenna system is characterized in that power and phase adjustment ofeach antenna module is possible because each of a plurality of antennamodules includes a RF module including a power amplifier, that is, anactive element.

As a general MIMO antenna structure, a linear antenna array, that is,one-dimensional antenna array, such as a uniform linear array (ULA), wasconsidered. In the one-dimensional array structure, beams which may beformed by beamforming are present in a two-dimensional plane. This isapplied to a passive antenna system (PAS)-based MIMO structure of anexisting base station. Although vertical antennas and horizontalantennas are present even in a PAS based base station, the verticalantennas are fixed to one RF module and thus beamforming is impossiblein a vertical direction and only mechanical tilting is applicable.

However, as an antenna structure of a base station has evolved to anactive antenna system, independent RF modules may be implemented invertical antennas and thus beamforming is possible not only in ahorizontal direction but also in a vertical direction. This is referredto as vertical beamforming or elevation beamforming.

According to vertical beamforming, since formable beams may be expressedin three-dimensional space in vertical and horizontal directions,vertical beamforming may be referred to as three-dimensionalbeamforming. That is, three-dimensional beamforming becomes possible byevolution from a one-dimensional antenna array structure to atwo-dimensional planar antenna array structure. Three-dimensionalbeamforming is possible not only in a planar antenna array structure butalso in a ring-shaped three-dimensional array structure.Three-dimensional beamforming is characterized in that a MIMO process isperformed in a three-dimensional space because various antennastructures may be used in addition to the one-dimensional antenna arraystructure.

FIG. 10 is a diagram showing an example of forming a UE-specific beambased on an active antenna system. Referring to FIG. 10, due tothree-dimensional beamforming, beamforming is possible not only when aUE moves from side to side but also when a UE moves back and forth,thereby providing a higher degree of freedom to UE-specific beamforming.

Further, as a transmission environment using a two-dimensional antennaarray structure based on an active antenna, an environment in which anoutdoor eNB transmits a signal to an outdoor UE, an environment in whichan outdoor eNB transmits a signal to an indoor UE (outdoor to indoor;O2I) and an environment in which an indoor eNB transmits a signal to anindoor UE (indoor hotspot) may be considered.

FIG. 11 is a diagram showing a 3-dimensional (3D) beam transmissionscenario based on an active antenna system.

Referring to FIG. 11, in an actual cell environment in which a pluralityof buildings is present per cell, an eNB needs to consider vertical beamsteering capabilities considering various UE heights due to buildingheights as well as UE-specific horizontal beam steering. In such a cellenvironment, channel properties different from those of an existingradio channel environment, e.g., shadow/path loss change due to heightdifference, fading property change, etc. need to be applied.

In other words, three-dimensional beamforming is evolved from horizontalbeamforming based on a one-dimensional linear antenna array structureand refers to a MIMO processing scheme which is an extension to or acombination with elevation beamforming or vertical beamforming based onan antenna structure of a multi-dimensional array, such as a planarantenna array, or a massive antenna array.

The massive antenna array has one or more of the followingcharacteristics. That is, i) the massive antenna array is located on atwo-dimensional (2D) plane or in a 3D space, ii) the number of logicalor physical antennas is eight or more (here, the logical antenna may beexpressed by an antenna port) and iii) each antenna is composed of anAAS. However, definition of the massive antenna array is not limitedthereto. Hereinafter, various beamforming schemes using a massiveantenna array will be described.

a) Partial antenna array based beamforming applied to a 3D beamformingenvironment is referred to as beam-width adaptation (BA) beamforming,which has the following features.

In the BA beamforming scheme, the number of antennas participating indata transmission is adjusted according to the speed of a UE to adjust atransmission beam width. FIG. 12 is a diagram showing comparison in beamcoverage between an existing MIMO transmission scheme and a BAbeamforming scheme. In particular, the left side of FIG. 12 shows theexisting MIMO transmission scheme and the right side thereof shows theBA beamforming scheme.

Referring to the left side of FIG. 12, in a 4×4 antenna array, if a UEmoves at a medium speed, the width of a beam transmitted by the 4×4antenna array is too narrow to obtain channel accuracy. Since anopen-loop scheme covers whole cell coverage, the beam width may beexcessively wide. As shown in the right side of FIG. 12, if only two 2×2central antenna arrays participate in transmission, a beam having arelatively wide beam width and capable of obtaining beam gain may begenerated. That is, the number of antennas participating in transmissionto the UE is reduced according to the speed of the UE to increase thebeam width, thereby acquiring beam gain lower than that of closed-loopbeamforming but higher than that of open-loop beamforming.

b) If the beam width is adjusted according to mobility of the UE in theBA beamforming scheme, a method for performing beamforming in a verticalor horizontal direction according to the movement direction of the UEand performing open loop precoding may also be considered. Thistechnology is referred to as dimension adaptation (DA) beamformingbecause 2D beamforming may be performed in a 3D beamforming environment.

The DA beamforming scheme is a beamforming scheme for, at an eNB,applying an open-loop scheme to the direction, in which movement of theUE is big, that is, the direction, in which the Doppler effect is high,of the vertical direction and the horizontal direction and applying aclosed-loop scheme to the other direction. FIG. 13 is a diagram showingthe concept of a DA beamforming scheme. In particular, the left side ofFIG. 13 shows the case in which a UE moves in a horizontal direction andthe right side thereof shows the case in which a UE moves in a verticaldirection.

FIG. 14 is a diagram showing the features of a DA beamforming scheme.

If a DA beamforming scheme is used, beam gain can be obtained in adirection in which the Doppler effect is low but cannot be obtained in adirection in which the Doppler effect is high. Accordingly, in an areain which a beam is generated, a beam having a narrow width is generatedin one of a horizontal direction and a vertical direction as shown inFIG. 14. Accordingly, it is possible to provide beam gain having apredetermined level to a UE moving in a specific direction.

c) Dimension and beam-width adaptation (DBA) which is a combination of aBA beamforming scheme and a DA beamforming scheme may also beconsidered. FIG. 15 is a diagram showing the concept of a DBAbeamforming scheme.

The DBA beamforming scheme is a combination of a DA beamforming schemeand a BA beamforming scheme. Referring to FIG. 15, if a UE moves in avertical or horizontal direction upon applying the DBA beamformingscheme, closed-loop beamforming is performed in a direction in which theDoppler effect is low, that is, in a direction orthogonal to movement ofa UE, and the number of antennas participating in transmission isadjusted according to the speed of the UE to adjust a beam width in adirection in which the Doppler effect having a predetermined level ispresent.

In summary, as shown in Table 1, the DA beamforming scheme is suitablewhen a UE moves at a high speed in a specific direction with respect toan eNB, the BA beamforming scheme is suitable when a UE moves at a lowspeed or a medium speed, and the DBA beamforming scheme is suitable whena UE moves in a specific direction at a low speed or a medium speed.

TABLE 1 Dimension adaptation A UE moves at a high speed in a vertical or(DA) beamforming horizontal direction with respect to an eNB. Beam-widthadaptation Low-speed or medium-speed movement beamforming environmentDBA beamforming A UE moves in a vertical or horizontal direction (DA +BA) with respect to an eNB at a low speed or a medium speed.

In order to adaptively apply a beamforming scheme such as a DA/BA/DBAbeamforming scheme according to channel variation, it is important tocheck whether a channel between an eNB and a UE is rapidly varied. Inparticular, for DA beamforming or DBA beamforming, both channelvariation in a vertical direction and channel variation in a horizontaldirection should be checked.

Accordingly, channel variation per unit time may be measured andreported by tracking variation in beamforming direction capable ofmaximizing channel gain.

In a 3GPP LTE system or a WiMax system, a UE estimates a MIMO channeland feeds PMI in a preferred beamforming direction back. At this time,PMI variation of a UE with time may mean channel variation. That is, ifpreferred PMI is rapidly varied, a channel environment is rapidly variedand, if preferred PMI is slowly varied, a channel environment is slowlyvaried. As a result, channel variation may be estimated via PMItracking.

In PMI tracking, a method for, at an eNB, estimating channel variationusing periodic/aperiodic PMI feedback information of a UE only may beconsidered. However, this method has the following limits.

1. Absence of Initial PMI History Information

That is, upon initial PMI reporting or upon aperiodic PMI reporting,there is a limit such as absence of a PMI history. In addition, evenwhen a UE is switched from an open-loop transmission mode to aclosed-loop transmission mode, there may be a limit such as absence of aPMI history.

2. Channel Variation Cannot be Measured within a PMI Reporting Period

That is, when a PMI reporting period is X msec, it is difficult tomeasure channel variation in X msec.

3. Tracking Accuracy Limit According to PMI Codebook Size

More specifically, tracking accuracy is low in a state in which the sizeof a PMI codebook, that is, the number of bits allocated to PMIfeedback, is restricted.

4. Difficulty in Checking Channel Variation Via PMI Tracking Upon PMITransmission of Rank 2 or More

Lastly, PMI of rank 2 or more refers to a matrix composed of orthogonalbeamforming vectors corresponding in number to the rank. In this case,it is difficult to measure channel variation according to variation inPMI matrix.

In order to solve such problems, first, an eNB configures a pilot signalfor enabling a UE to measure a PMI variation value and feedback relatedthereto. Thereafter, the UE measures the PMI variation value withrespect to the pilot signal regardless of PMI feedback and performsfeedback related thereto.

More specifically, according to operation of a current LTE system, apilot signal (e.g., CSI-RS) is configured and, at the same time, whetherPMI is reported is determined in a PUCCH/PUSCH reporting mode.Accordingly, the UE configures the pilot signal and, at the same time,determines whether the PMI of the pilot signal is calculated andreported. However, when the PMI variation value is measured, the UEperforms a procedure of storing a PMI variation history with respect tospecific pilot signals and reporting related information separately withthe CSI reporting procedure.

The related feedback information may be configured using variousmethods. Examples of the feedback information related to PMI variationare as follows.

1) Variance or Standard Deviation of PMI During Specific Duration (SeeEquation 11 Below)

$\begin{matrix}{\frac{1}{T}{\sum\limits_{t = T_{0}}^{T_{0} + T - 1}\;\left( {{PMI}_{t} - {E\left\{ {PMI} \right\}}} \right)^{2}}} & \left\lbrack {{Equation}\mspace{14mu} 11} \right\rbrack\end{matrix}$

where,

${{E\left\{ {PMI} \right\}} = {{\frac{1}{T}{\sum\limits_{t = T_{0}}^{T_{0} + T - 1}\;{{PMI}_{t}\mspace{14mu}{or}\mspace{14mu} E\left\{ {PMI} \right\}}}} = {\frac{1}{T_{0} + T - 1}{\sum\limits_{t = 1}^{T_{0} + T - 1}\;{PMI}_{t}}}}},$t denotes a measurement time index, T0 denotes a measurement start timeand T denotes a measurement duration.

2) PMI Variation Value During Specific Duration (See Equation 12)PMI_(T) ₁ −PMI_(T) ₀   [Equation 12]

where, T0 denotes a reference time, T1 denotes a reporting time and isequal to T0+C (C being a constant).

3) PMI Variation Value to which a Weight is Applied (See Equation 13Below)

$\begin{matrix}{\sum\limits_{t = {T_{0} + 1}}^{T_{1}}\;{w_{t}\left( {{PMI}_{t} - {PMI}_{t - 1}} \right)}} & \left\lbrack {{Equation}\mspace{14mu} 13} \right\rbrack\end{matrix}$

where, wt denotes a weight at a time t, which may increase with time t.

4) PMI Average Variation Value (See Equation 14 Below)

$\begin{matrix}{\frac{1}{T}\left( {{\sum\limits_{t = T_{2}}^{T_{2} + T - 1}\;{PMI}_{t}} - {\sum\limits_{t = T_{0}}^{T_{0} + T - 1}\;{PMI}_{t}}} \right)} & \left\lbrack {{Equation}\mspace{14mu} 14} \right\rbrack\end{matrix}$

where, T₂+T₁=reporting time T1.

In the above examples, for correlation between the PMI variation valueand the channel variation, indices aligned in order of beamforming angleare used as PMI. If a PMI codebook used in an actual LTE system isreused, since the codebook is not aligned in order of beamforming angle,codebook permutation or rearrangement for rearranging the codebook inorder of beamforming angle is further required.

For example, if existing codebook indices are arranged in order ofbeamforming angle of a uniform linear array (ULA), that is, in order of0 degrees, 90 degrees, −90 degrees, 30 degrees and −30 degrees, aprocess of rearranging the indices in order of −90 degrees, −30 degrees,0 degrees, 30 degrees and 90 degrees is required. Such a rearrangementprocess applies to only a PMI codebook for preferred beam tracking butdoes not apply to a PMI codebook for existing CSI feedback.

In order not to perform the rearrangement process, as in the followingembodiment, a beamforming angle value indicated by PMI may be directlyused. That is, as shown in Equation 15 below, a beamforming anglevariation value of PMI during a specific duration is used as feedbackinformation.Angle(PMI_(T) ₁ )−Angle(PMI_(T) ₀ )  [Equation 15]

where, T0 denotes a reference time, T1 denotes a reporting time and T1is equal to T0+C (C being a constant). In addition, Angle(x) means abeamforming angle corresponding to a PMI index x.

Meanwhile, a PMI codebook for measuring a PMI variation value is newlydefined or an existing PIM codebook is reused. At this time, i) thecodebook is configured in order of beamforming angle, ii) the codebookis composed of rank-1 PMI only, iii) the codebook is elaborate, that is,the size of the codebook is greater than that of an existing codebook,and iv) existing rank-1 PMI is included. Here, features ii) and iii) cansolve the above-described tracking accuracy problem and ambiguity inchannel variation measurement due to PMI tracking in case of rank 2 ormore.

Additionally, in feature iv), since transmission is performed based onthe existing rank-1 PMI codebook upon closed-loop MIMO transmission viaa single layer, the existing rank-1 PMI codebook is preferably includedeven upon channel variation measurement in consideration of an actualbeamforming pattern. However, if some PMI is included in the existingrank-1 PMI codebook for purposes other than different antennaarrangement (e.g., cross polarization) or beamforming (e.g., for thepurpose of selecting an antenna), this PMI may be excluded from thecodebook for measuring the PMI variation value.

According to the above description, the problem that the initial PMIhistory is absent and the problem that the channel variation cannot bemeasured within the PMI reporting period can be solved. For example,when a CSI-RS is transmitted at a period of 4 msec, PMI reporting isperformed at a period of 20 msec by independent pilot and feedbackconfiguration but PMI history tracking is performed at a period of 4msec. In this case, since feedback of the PMI history does not need tobe performed at a period of 4 msec, feedback may be performed at alonger period such that feedback overhead is not increased. That is, themeasurement period and the reporting period of the PMI variation valuemay be different from each other.

The PMI variation value may be aperiodically reported. That is, when thePMI is varied by a predetermined value or more, that is, if the channelvariation is equal to or greater than a predetermined value, reportingmay be performed. Alternatively, if the channel variation is not large,feedback is periodically performed and, if the channel variation isequal to or greater than the predetermined value, feedback of the PMIvariation value is performed at a short period.

Accordingly, the UE may report the measured PMI variation value to theeNB at a predetermined period or only when the PMI variation value isequal to or greater than the predetermined value. Alternatively, the UEmay determine the reporting period of the measured PMI variation valueaccording to the PMI variation value.

A method of utilizing the above-described technology for measurement ofthe channel variation in the vertical and horizontal directions for DAbeamforming or DBA beamforming will now be described.

First, a pilot for measuring a PMI variation value is divided into apilot for channel variation measurement in a vertical direction and apilot for channel variation measurement in a horizontal direction asshown in FIGS. 16 to 19.

FIGS. 16 and 17 are diagrams showing examples of a pilot for channelmeasurement in a vertical direction according to an embodiment of thepresent invention. Referring to FIGS. 16 and 17, the pilot for channelvariation measurement in the vertical direction is characterized in thatantenna ports are sequentially mapped in the vertical direction of anantenna array.

FIGS. 18 and 19 are diagrams showing examples of a pilot for channelmeasurement in a horizontal direction according to an embodiment of thepresent invention. Referring to FIGS. 18 and 19, the pilot for channelvariation measurement in the horizontal direction is characterized inthat antenna ports are sequentially mapped in the horizontal directionof an antenna array.

If the pilot for channel variation measurement in the vertical directionand the pilot for channel variation measurement in the horizontaldirection are respectively transmitted using the above-describedmethods, the UE may measure and report the PMI variation values of thepilots so as to measure channel variation in the vertical direction andchannel variation in the horizontal direction.

In application of the above-described PMI tracking technology, even whenthe pilot for channel variation measurement in the vertical directionand the pilot for channel variation measurement in the horizontaldirection are simultaneously transmitted, the UE may measure and reporta variation value of vertical PMI (hereinafter, referred to as V-PMI)and horizontal PMI (hereinafter, referred to as H-PMI) so as to measurechannel variation in the vertical direction and channel variation in thehorizontal direction.

The PMI tracking technology is valid if the number of PMI candidates issufficient but may not have a desired level of accuracy if the number ofPMI candidates is not sufficient due to restriction on a feedbackchannel. For example, if beam accuracy is set such that a beamformingangle is 10 degrees or more per PMI, it is difficult to measure channelvariation by measuring the PMI variation only.

In order to solving these problems, it is proposed a method formeasuring and reporting channel quality variation for performing channelvariation measurement more accurately. The method for measuring andreporting channel quality variation may be used together with theabove-described PMI tracking method.

First, the UE measures a channel quality variation value of a specificPMI set and performs feedback related thereto. The channel qualityvariation value may be defined with respect to each or all of PMIincluded in the PMI set. Channel quality may apply to a channel value(index), channel gain (index), channel quality (index), etc. The PMI setmay be defined as a whole PMI set or a PMI set configured by a network.Additionally, the PMI set may be defined as a PMI set composed of N PMIselected by the UE, that is, a UE-selective PMI set.

In particular, the PMI set selected by the UE may include PMI determinedbased on receive power, received signal quality, signal to noise plusinterference ratio (SINR), CQI, etc. and N may be predetermined orspecified by the network. Alternatively, the number N of PMI included inthe PMI set may be determined according to received signal quality.

Even when the PMI is not varied during a specific duration, since achannel is varied during the specific duration, the present methodmeasures and reports channel variation. Such variation is measured withrespect to a specific PMI set and such a PMI set may be defined usingvarious methods as described above.

FIG. 20 is a diagram showing an example of calculating a channelvariation value during a specific duration.

Referring to FIG. 20, if a UE-selective PMI set with N of 2 is used, PMI3 and PMI 4 may be included in the PMI set based on the value measuredat t=T0. Hereinafter, PMI elements included in the PMI are {PMI 1, PMI2, . . . , PMI N} and the channel quality value of PMI i at a time t isQ(i, t).

If only one channel variation value is defined with respect to the PMIset, the channel variation value may be defined using various methods asfollows.

A) Secondary Momentum Related Value of Channel Quality During a SpecificDuration, Such as Variance or Standard Deviation (See Equation 16 Below)

$\begin{matrix}{\frac{1}{N}{\sum\limits_{i = 1}^{N}\;{\sum\limits_{t = T_{0}}^{T_{1}}\;\left\{ {{Q\left( {i,t} \right)} - {E_{t}\left\{ {Q\left( {i,t} \right)} \right\}}} \right\}^{2}}}} & \left\lbrack {{Equation}\mspace{14mu} 16} \right\rbrack\end{matrix}$

where, E_(i){x} denotes an average of X at time t, t denotes ameasurement time index, T0 denotes a measurement start time, T1 denotesa measurement end time, and i denotes an index of each PMI element inthe PMI set.

B) Channel Quality Variation Value During a Specific Duration (SeeEquation 17 Below)

$\begin{matrix}{\sum\limits_{i = 1}^{N}{{{{Q\left( {i,T_{1}} \right)} - {Q\left( {i,T_{0}} \right)}}}\mspace{14mu}{or}\mspace{14mu}{\sum\limits_{i = 1}^{N}{{{Q\left( {i,T_{1}} \right)} - {Q\left( {i,T_{0}} \right)}}}^{2}}}} & \left\lbrack {{Equation}\mspace{14mu} 17} \right\rbrack\end{matrix}$

C) Channel Quality Variation Value to which a Weight is Applied (SeeEquation 18 Below)

$\begin{matrix}{\sum\limits_{t = {T_{0} + 1}}^{T_{1}}\;{w_{t}{\sum\limits_{i = 1}^{N}{{{Q\left( {i,t} \right)} - {Q\left( {i,{t - 1}} \right)}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 18} \right\rbrack\end{matrix}$

where, wt denotes a weight at a time t, which may increase with t.

D) Variation Value of a Channel Quality Average (See Equation 19 Below)

$\begin{matrix}{\frac{1}{T}{\sum\limits_{i = 1}^{N}\left( {{\sum\limits_{t = T_{2}}^{T_{2} + T - 1}{Q\left( {i,t} \right)}} - {\sum\limits_{t = T_{0}}^{T_{0} + T - 1}{Q\left( {i,t} \right)}}} \right)}} & \left\lbrack {{Equation}\mspace{14mu} 19} \right\rbrack\end{matrix}$

where, T denotes a measurement duration, that is, a window size, andT2+T−1 denotes a reporting time T1.

Feedback of the channel quality variation value may be performed at aperiod specified by the eNB or may be performed only when the channelquality variation value is equal to or greater than a predeterminedvalue or the reporting period may be determined according to the channelquality variation value.

In order to use the proposed technology to measure channel variation inthe vertical and horizontal directions for DA beamforming or DBAbeamforming, channel quality variation values of a pilot for channelvariation measurement in the vertical direction and a pilot for channelvariation measurement in the horizontal direction may bemeasured/reported. For example, when the eNB includes a rectangular N×Mantenna array and each antenna transmits a pilot, the UE may estimate a1×M channel from the pilot signals transmitted at a specific row andmeasure/report a channel quality variation value of an H-PMI set of thechannel. In addition, the UE may estimate a 1×N channel from the pilotsignals transmitted at a specific column and measure/report a channelquality variation value of a V-PMI set.

Although a method for measuring channel variation of one row or columnin the vertical/horizontal direction is described in the above example,an average of channel variation values of a plurality of rows or columnsmay be measured/reported.

Although the proposed technology is described on the assumption that PMIfeedback is performed after receiving a non-precoded pilot, the presentinvention is not limited thereto. For example, the eNB may transmit aplurality of precoded pilots as a reference signal. In this case, theabove-described PMI tracking technology is applicable using a preferredpilot index variation value instead of a PMI variation value.

More specifically, the eNB configures a pilot signal for enabling the UEto measure the PPI variation value and feedback related thereto. The UEmeasures the PPI variation value and performs feedback related thereto.In particular, the UE reports the measured PPI variation value at aperiod specified by the eNB or only when the PPI variation value isequal to or greater than a predetermined value or the reporting periodmay be determined according to the PPI variation value.

In order to measure channel variation in the vertical/horizontaldirection based on a plurality of precoded pilots, the PPI variationvalue may be measured/reported with respect to a pilot index of thevertical/horizontal direction. This will be described with reference tothe drawings.

FIG. 21 is a diagram showing an example of measuring and reporting apilot index in a vertical/horizontal direction.

Referring to FIG. 21, if a reference signal to which a vertical precoderv and a horizontal precoder h are applied is r(v, h), the UE measuresand reports a variation value of a preferred reference signal withrespect to the vertical precoder v and the horizontal precoder h. Forexample, if a reference signal having best signal quality at a time T0is r(v0, h0) and a reference signal having best signal quality at a timeT1 is r(v1, h1), the PPI variation value in the vertical direction maybe |v₁−v₀| and the PPI variation value in the horizontal direction maybe |h₁−h₀|. This is a method for measuring the channel variation in thevertical and horizontal directions via variation in pilot index havingbest signal quality.

The above-described technology for measuring channel variation based onthe channel quality variation value may be achieved based on a pluralityof non-precoded pilots. In this case, the same method is applicableexcept that a pilot set is used instead of the PMI set.

FIG. 22 is a diagram showing an example of measuring and reportingchannel variation in a vertical direction and a horizontal directionbased on a non-precoded pilot.

Referring to FIG. 22, in order to measure channel variation in thevertical and horizontal directions based on the non-precoded pilot, theUE measures channel variation with respect to pilots to which the samehorizontal/vertical precoder is applied. At this time, pilot sets forchannel variation measurement may be separately configured.

In particular, in FIG. 22, an example of configuring the pilot set forchannel measurement in the horizontal/vertical direction based on thePPI if the PPI is not varied with time and measuring channel variationtherefrom is shown. More specifically, it is assumed that the pilot setis composed of three pilots having excellent quality from the viewpointof the UE and an environment in which channel variation in thehorizontal direction is relatively large is assumed.

Although the method for measuring channel variation in thevertical/horizontal direction based on the measured value of one fixedvertical/horizontal precoded pilot is proposed in the above example, anaverage of variation values measured at a plurality of rows or columnsmay be measured/reported.

In the above-described method, if signal quality of the fixedvertical/horizontal precoded pilot deteriorates (e.g., the direction isa null direction), a problem may occur in measurement accuracy. In orderto solve such a problem, the eNB may transmit a precoded pilot in whichbeam sharpness is decreased in a specific direction.

More specifically, the eNB transmits a precoded pilot to have a narrowbeam width in any one of the vertical direction or the horizontaldirection and the UE measures a channel quality variation value in eachdirection.

FIG. 23 is a diagram showing another example of measuring and reportinga pilot index in a vertical/horizontal direction.

Referring to FIG. 23, in order to measure channel variation in thevertical direction, a plurality of pilots is transmitted using antennascorresponding to a specific column of an antenna array only. In thiscase, since sharp beams are precoded and transmitted in differentdirections according to the pilot signal in the vertical direction butthe same precoding is applied to all pilot signals to have a wide beamwidth in the horizontal direction, when the UE measures qualityvariation of each pilot signal, it is possible to measure channelvariation in the vertical direction.

Similarly, by generating a beam which is sharp in the horizontaldirection but has a wide beam width in the vertical direction usingantennas corresponding to a specific row and transmitting a precodedpilot, it is possible to measure channel variation in the horizontaldirection.

In application of the present technology, parameters such as channelquality, measurement duration T for PMI or PPI variation, measurementstart time T1 and reporting time T2 may be prescribed as standard, maybe configured by the network and delivered to the UE via RRC signaling,or may be determined by the UE.

The present invention proposes a method for designing a pilot signal ora reference signal for channel change measurement in an LTE system.

The reference signal for channel change measurement proposed by thepresent invention is differentiated from a CSI-RS or CRS which is areference signal for channel state information measurement in terms ofthe following features.

1) The reference signal for channel change measurement is precoded byrecoders of rank 1, which differ between antenna ports. Of course, theprecoders of rank 1 should be predetermined in both a transmitter and areceiver.

2) Antenna ports of the reference signal for channel change measurementare mapped to REs for a PDSCH in legacy release and transmitted from abase station to a UE.

3) Lastly, the reference signal for channel change measurement hasfrequency density lower than that of a conventional CSI-RS or CRS withina frequency bandwidth. For example, in the overall frequency band, thereference signal for channel change measurement preferably has frequencydensity of one RE or less per RB. Since the reference signal for channelchange measurement is mainly used for the purpose of channel changemeasurement rather than instantaneous channel information such as aCQI/PMI/RI, the reference signal for channel change measurement may bedesigned to have very low frequency density. For example, antenna portsof the reference signal for channel change measurement may be configuredto be mapped to only specific RBs of the overall frequency band andtransmitted. In particular, since the reference signal for channelchange measurement may be commonly received by users in a cell, resourceoverhead may be low.

Additionally, the reference signal for channel change measurementaccording to the present invention is preferably transmitted via some ofall PRBs. At this time, the reference signal for channel changemeasurement is not transmitted via a resource (PRB and subframe) viawhich a primary synchronization signal (PSS), a secondarysynchronization signal (SSS) and a physical broadcast signal (PBCH) aretransmitted

FIG. 24 is a diagram showing an example of transmitting a referencesignal for channel change measurement according to an embodiment of thepresent invention. In particular, FIG. 24 shows an example oftransmitting a reference signal for channel change measurement(hereinafter, referred to as a special RS) via one specific RB.

Of course, 6 center RBs via which a PSS, an SSS and a PBCH aretransmitted may be excluded from RB locations for the specific RS. Inaddition, in order to prevent collision between the special RSs of eNBs,the RB location for the special RS may be differently set according toeNB. The UE may receive the RB location for the special RS via RRCsignaling. Alternatively, code division multiplexing (CDM) may bepossible by fixing the RB location for the special RS and usingdifferent scrambling identifiers for the same antenna ports transmittedby different eNBs. Alternatively, frequency division multiplexing (FDM)and/or time division multiplexing (TDM) may be possible by fixing the RBlocation for the special RS and mapping the same antenna portstransmitted by different eNBs to different RE locations.

Although different antenna ports are mapped to different REs and a totalof 23 antenna ports is transmitted using a first slot in FIG. 24, thisis only exemplary and the present invention is not limited thereto.

Meanwhile, a reference signal for channel change measurement accordingto the present invention, that is, a special RS, may be transmittedusing one or both of a first slot and a second slot on the time axis. Ifthe special RS is transmitted using one of the first slot and the secondslot, a PDSCH may be transmitted using the slot using which the specialRS is not transmitted. That is, PDSCH resource mapping may be performedin slot units.

If the special RS is transmitted using both slots, as shown in FIG. 24,more antenna ports may be designed to be transmitted while time-sidedensity of the special RS is maintained at one antenna port persubframe. For example, 23 antenna ports are transmitted using the firstslot and 28 antenna ports are transmitted using the second slot.Alternatively, the same antenna ports may be repeatedly transmitted inslot units, thereby increasing channel estimation accuracy or moreaccurately tracking channel change with the elapse of time.

FIG. 25 is a diagram showing an example of repeatedly transmitting anantenna port of a reference signal for channel change measurement at thesame time interval according to an embodiment of the present invention.

Referring to FIG. 25, it can be seen that the same antenna port istransmitted using the same subframe at an interval of 7 OFDM symbols. Inthis case, since antenna ports #0 to antenna port #11 of the special RSare transmitted at locations where resources for CSI-RSs are configured,the CSI-RSs may not be transmitted using the resources for the CSI-RSsand interference may be caused by the CSI-RSs of the neighboring cells.However, since the same antenna port is always transmitted using thesame subcarrier at the same time interval, tracking performance for UEmotion becomes better.

In particular, in FIG. 25, a PDSCH may be transmitted at an empty RElocation or a special RS antenna port starting from antenna port #24 maybe further defined and transmitted.

In the latter case, antenna port #0 to antenna port #23 and antenna port#24 are different in terms of transmission density of the antenna port,a beamforming value applied to each antenna port may be set according tothe transmission density of the antenna port. More specifically, abeamforming angle (e.g., −X degrees to +X degrees) for a boresight sidehaving relatively high performance sensitivity may be allocated to anantenna port having high transmission density and a beamforming angle(e.g., −Y degrees to −X degrees, +X degrees to +Y degrees) (here, Ybeing a maximum beamforming angle of an antenna array) for a side havingrelatively low performance sensitivity may be allocated to an antennaport having low transmission density.

As another example, an antenna port having high transmission density maybe allocated for motion measurement in a direction (e.g., a horizontaldirection) having relatively high sensitivity for UE motion upon 3Dbeamforming and an antenna port having low transmission density may beallocated for motion measurement in a direction having low sensitivity.Alternatively, UE motion may be statistically analyzed and then anantenna port having higher density may be allocated to a direction inwhich UE actively moves. The above-described antenna port mapping methodis only exemplary and various antenna port mapping methods may beconsidered. In particular, antenna ports are mapped to locations where aphysical channel or physical signal may be transmitted and resourcemapping prioritization may be performed if collision therebetweenoccurs.

The special RS proposed by the present invention is mainly used toestimate UE mobility but the present invention is not limited thereto.The special RS may replace PMI feedback in a small cell environmenthaving relatively low delay spread or in an environment having arelatively small frequency bandwidth to feed back a preferred antennaport index, thereby indirectly feeding back a precoder applied thereto.Alternatively, the special RS may be used for beam fine tuning for PMIfeedback. Alternatively, the special RS may be used for layer 3measurement of RSRP/RSRQ.

FIG. 26 is a block diagram of a communication apparatus according to oneembodiment of the present invention.

Referring to FIG. 26, a communication apparatus 2600 includes aprocessor 2610, a memory 2620, a Radio Frequency (RF) module 2630, adisplay module 2640 and a user interface module 2650.

The communication apparatus 2600 is shown for convenience of descriptionand some modules thereof may be omitted. In addition, the communicationapparatus 2600 may further include necessary modules. In addition, somemodules of the communication apparatus 2600 may be subdivided. Theprocessor 2610 is configured to perform an operation of the embodimentof the present invention described with reference to the drawings. For adetailed description of the operation of the processor 2610, referencemay be made to the description associated with FIGS. 1 to 25.

The memory 2620 is connected to the processor 2610 so as to store anoperating system, an application, program code, data and the like. TheRF module 2630 is connected to the processor 2610 so as to perform afunction for converting a baseband signal into a radio signal orconverting a radio signal into a baseband signal. The RF module 2630performs analog conversion, amplification, filtering and frequencyup-conversion or inverse processes thereof. The display module 2640 isconnected to the processor 2610 so as to display a variety ofinformation. As the display module 2640, although not limited thereto, awell-known device such as a Liquid Crystal Display (LCD), a LightEmitting Diode (LED), or an Organic Light Emitting Diode (OLED) may beused. The user interface module 2650 is connected to the processor 2610and may be configured by a combination of well-known user interfacessuch as a keypad and a touch screen.

The above-described embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional on the condition thatthere is no additional remark. If required, the individual constituentcomponents or characteristics may not be combined with other componentsor characteristics. In addition, some constituent components and/orcharacteristics may be combined to implement the embodiments of thepresent invention. The order of operations disclosed in the embodimentsof the present invention may be varied. Some components orcharacteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary. Moreover, it will be apparent that some claims referring tospecific claims may be combined with other claims referring to the otherclaims other than the specific claims to constitute the embodiment oradd new claims by means of amendment after the application is filed.

In this document, a specific operation described as performed by the BSmay be performed by an upper node of the BS. Namely, it is apparentthat, in a network comprised of a plurality of network nodes including aBS, various operations performed for communication with a UE may beperformed by the BS, or network nodes other than the BS. The term BS maybe replaced with the terms fixed station, Node B, eNode B (eNB), accesspoint, etc.

The embodiments of the present invention can be implemented by a varietyof means, for example, hardware, firmware, software, or a combinationthereof. In the case of implementing the present invention by hardware,the present invention can be implemented through application specificintegrated circuits (ASICs), digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), a processor, a controller, amicrocontroller, a microprocessor, etc.

If operations or functions of the present invention are implemented byfirmware or software, the present invention can be implemented in theform of a variety of formats, for example, modules, procedures,functions, etc. The software code may be stored in a memory unit so asto be driven by a processor. The memory unit may be located inside oroutside of the processor, so that it can communicate with theaforementioned processor via a variety of well-known parts.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

INDUSTRIAL APPLICABILITY

Although an example in which a method and apparatus for transmitting areference signal for channel change measurement in a wirelesscommunication system is applied to a 3GPP LIE system has been described,the present invention is applicable to various wireless communicationsystems in addition to the 3GPP LTE system.

The invention claimed is:
 1. A method for transmitting a referencesignal from a base station to a user equipment (UE) in a wirelesscommunication system, the method comprising: mapping, by the basestation, the reference signal defined by a plurality of antenna ports toresource elements; applying, by the base station, precoding to thereference signal using different precoders according to the plurality ofantenna ports; and transmitting, by the base station, the precodedreference signal to the UE, wherein mapping the reference signalcomprises: mapping the reference signal defined by the plurality ofantenna ports to resource elements included in a first slot of onesubframe; and repeatedly mapping the reference signal defined by theplurality of antenna ports to resource elements included in a secondslot of the subframe wherein: a resource element is defined by a symbolindex and a subcarrier index, and for each of the plurality of antennaports, subcarrier indices of the resource elements of the first slot, towhich the reference signal is mapped, are equal to subcarrier indices ofthe resource elements of the second slot, to which the reference signalis mapped.
 2. The method according to claim 1, wherein the precoderscorrespond to rank
 1. 3. The method according to claim 1, wherein theresource elements are included in only one of all resource blocksallocated to the UE.
 4. The method according to claim 3, whereininformation about the resource block is transmitted to the UE via ahigher layer signal.
 5. A transmitter in a wireless communication systemcomprising: a processor configured to map a reference signal defined bya plurality of antenna ports to resource elements and apply precoding tothe reference signal using different precoders according to theplurality of antenna ports; and a radio frequency unit configured totransmit the precoded reference signal to a receiver, wherein theprocessor maps the reference signal defined by the plurality of antennaports to resource elements included in a first slot of one subframe, andrepeatedly maps the reference signal defined by the plurality of antennaports to resource elements included in a second slot of the subframe,wherein: a resource element is defined by a symbol index and asubcarrier index, and for each of the plurality of antenna ports,subcarrier indices of the resource elements of the first slot, to whichthe reference signal is mapped, are equal to subcarrier indices of theresource elements of the second slot, to which the reference signal ismapped.
 6. The transmitter according to claim 5, wherein the precoderscorrespond to rank
 1. 7. The transmitter according to claim 5, whereinthe resource elements are included in only one of all resource blocksallocated to the receiver.
 8. The transmitter according to claim 7,wherein information about the resource block is transmitted to thereceiver via a higher layer signal.